If he goes longer than 15 he can’t return at all - he’s permanently off as an injury.
I'm trying to work out the timeline. The TRC and ESPNscrum sites both show White leaving the field for Ikitau in the 68th minute. That isn't correct. Ikitau is on the field at the same as White from 65th minute after the Harry Wilson try.
I think what happened is that Ikitau returns to the field right around the 15 minute time limit and McDermott leaves the field. McDermott then comes back onto the field after the Jorgensen try to replace White.
On that basis the blood injury returning was effective so Edmed could have still be substituted on at the end of the match.